Physicists have designed the world’s smallest refrigerator, small enough that it can’t hold any of your food. The fridge consists of three qubits–quantum particles that act as on-off switches. These quantum particles could be ions, atoms, or subatomic particles.

Other small systems have been created, but this is the first that doesn’t rely on external mechanisms, such as sophisticated lasers. “The whole guts of the fridge, it’s all accounted for and not hidden in some macroscopic object which is really doing the work,” [coauthor Noah] Linden says. [Science News]

Kitchen refrigerators work by shuttling heat away from one area (where you store your food) and dumping it somewhere else (the coils behind). This transfer isn’t news. Fans of thermodynamics have built devices to wick away heat from one source and dump it somewhere else since the nineteenth century. The device proposed in a paper to appear in Physical Review Letters uses the same basic technique but at a much smaller scale–on the size of three qubits, connected to two “baths,” one cold (or around room temperature) and one hot.

You could represent an excited qubit as a 1, and an unexcited qubit as a 0. The trio might start in a state of “101” which means “excited unexcited excited” or “010” which means “unexcited excited unexcited.” Since the three qubits are linked, the system could flip between the two setups. You might think of the excited qubits passing their excitement, or transferring energy, to the unexcited qubits–until the excited qubits become unexcited and the unexcited qubits become excited. If they are all the same temperature, they can flip back and forth from 101 to 010 to 101… all the live long day.

But this equal flipping changes when you add the hot and cold baths:

The trick is to put the first two qubits in contact with a cold bath and the third one in contact with a hot bath. The higher temperature makes it more likely that the third qubit will be in its excited state—and thus that the trio will be in the (101) state instead of the (010) state. But that means the system is more likely to flip out of (101) and into (010) than the other way around. So on average the flipping takes the first qubit from its excited state to its ground state and draws energy out of the first qubit. After a flip, the qubits essentially reset by interacting with the baths, allowing the cycle to start again. [ScienceNOW]

The researchers say they can just leave the system be and it will continue drawing energy out of that first qubit as long as the hot bath remains hot. Besides serving as an interesting look at thermodynamics on a quantum scale, the researchers believe the setup, when built, may find use in quantum computing.

Though one possibility for actually constructing the very mini-fridge involves capturing ions to act as qubits, ScienceNOWreports, an even smaller hypothetical fridge uses a particle with three levels instead of two.

Linden and his team also propose an even smaller system, in which a single particle with three distinct levels of quantum information, called a qutrit, acts as the refrigerator. “We believe this is the smallest possible thing you can call a fridge,” Linden says. [Science News]

Though quantum theorists may now hold the title for smallest refrigerator, particle physicists’ Large Hadron Collider with its superfluid helium “cryogenic distribution system” can cool a 16 mile accelerator ring, arguably making the LHC the planet’s largest fridge.

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